As a method for removing nitrogen oxides in an exhaust gas, conventionally, a method of selective reduction with ammonia using a catalyst has been mainstream. As an apparatus therefor, there is being used an apparatus for denitration reaction, in which ammonia serving as a reducing agent is introduced, together with an exhaust gas, into a reactor packed with a catalyst layer having NOx removal activity and a flow path parallel to the exhaust gas.
When dust is contained in an exhaust gas as seen in coal-fired power generation, an electric dust collector or a bag filter is generally disposed on the downstream side of a NOx removal apparatus and an air preheater for dust removal, or an electric dust collector is disposed on the upstream side of the NOx removal apparatus in some cases for dust removal. In the case of the former, since dust flows directly into the reactor, in order to prevent abrasion of a catalyst layer, a cross-section of the reactor is set to be large and a flow rate of gas flowing into the catalyst layer is set to be low. In the case of the latter, since the exhaust gas is treated under high temperature conditions, a large-scale dust remover is necessary, and although abrasion of the catalyst is remarkably reduced due to the treatment after dust removal, remaining fine dust adheres to the catalyst layer, which becomes easily blocked. Therefore, countermeasures such that an exhaust gas flow path in the catalyst layer is set to be large are taken to increase the catalyst capacity.
On the other hand, with respect to the arrangement of a duct for introducing an exhaust gas to the reactor, the following countermeasures have been taken: a duct is provided with an vertical portion relative to a flow of an exhaust gas, and thereby conveyance of relatively large dust such as massive soot is inhibited and then soot is collected and discharged using a hopper disposed in the lower portion of the vertical portion. Further, countermeasures have been taken to provide a mesh screen (a filter) such as a metal mesh in a duct of the boiler outlet to collect dust having a particle diameter larger than a mesh opening thereof. For example, Patent Document 1 discloses methods, in which a duct cross-section is provided with a mesh-shaped screen having openings smaller than the spaces of various types of catalysts (as a reference example, the pitch is 5 mm and the wire diameter is 1 mm) to remove dust; and a louver-shaped plate is disposed in a duct to cause dust having large particle diameter to collide with the duct and to fall. However, since a flow rate of an exhaust gas is about 15 m/s in a smoke path on the downstream side of an economizer of the boiler, aged deterioration due to abrasion may occur. In addition, proposed is a method, in which a mesh screen such as a metal mesh and the like is disposed in an inlet of a reactor vertically to a gas flow, and a mesh opening of the screen is adjusted to collect dust larger than a catalyst flow path. However, this method has no dust removal function, resulting in the possibility of clogging over time.
Further, Patent Document 2 describes that since dust having large particle diameter tends to be deposited on the side of a reactor of the boiler side, a mesh or a porous plate for removing dust is disposed in the lower portion of two upper vanes to remove ash. However, in this case, a gas flow tends to be non-uniform.